Most families contain both widely distributed species and some with very limited ranges. Wide distribution in this sense refers to a zoogeographical or faunal region, limited distribution to a single valley, plain, island, altitude zone, or vegetation type on a mountain.

Diversity of structure among adult beetles is as great as range of size. The ground beetles (Carabidae) have a rather generalized (primitive) form—the flattened, oval body has a relatively even surface, with regular ridges or grooves; antennae and legs are of moderate length and slender. The underside of most water beetles (Hydrophilidae) is oval, smooth, and flattened, the antennae either short or very slender, and the forelegs short, the hindlegs long and fringed with hairs that are used as paddles. Rove beetles (rove beetle) (Staphylinidae) have very short elytra and a slender abdomen. Soldier beetles (soldier beetle) (Cantharidae), fireflies (Lampyridae (firefly)), and net-winged beetles (net-winged beetle) (Lycidae) have soft elytra.

Click beetles (click beetle) (Elateridae) have a hingelike joint in the body region called the thorax that enables them to snap their bodies and jump high in the air; their relatives, the Buprestidae (metallic wood-boring beetle) (metallic wood borers), cannot jump but take flight very quickly. Cleridae (checkered beetles) are usually oblong or cylindrical, fairly active, and often brightly coloured. Nitidulidae (sap beetles) are short, flattened, and have slightly shortened elytra. Coccinellidae (ladybugs, ladybird beetles) are rounded, with a smooth, raised upper surface and a flat underside. The Endomychidae (handsome fungus beetles) often have enlarged, rounded elytra. The Erotylidae (pleasing fungus beetles) are usually slender, smooth, and shiny, as are the Languriidae.

Among the stout or cylindrical lamellicorns (Scarabaeoidea) are a number of bizarre forms. The male rhinoceros beetles (Dynastinae) have one or more horns on the head and sometimes on part of the thorax. Many of the true scarabs (Scarabaeinae) and other dung-feeding groups of the lamellicorns also have horns, including some of the goliath beetles (Cetoniinae (flower chafer)). Male stag beetles (stag beetle) (Lucanidae) have greatly enlarged mandibles (jaws); some are as long as the rest of the body.

The Chrysomelidae (leaf beetles (leaf beetle)) vary from simple eggshaped forms to slender, flat, or wedge-shaped ones, with wide elytra in the tortoise beetles (Cassidinae) and often numerous spines or tubercles in the leaf-mining leaf beetles (Hispinae). The Bruchidae (pea weevils) are short and stumpy, with short stout legs. The head, slightly elongated in front, is similar to that of some curculionid weevils. The Cerambycidae (long-horned wood-boring beetles), diverse in form and structure, usually have antennae that are longer than the body. Cerambycids may be slender and medium to large in size or very small.

The extremely diverse Tenebrionidae (darkling beetles) are not always recognized as members of one family. Most arboreal (tree-dwelling) forms in the tropics or subtropics are slender and long legged. Some slender and egg-shaped forms have a metallic sheen; most of the ground-dwelling forms are black and robust. The large, tropical Trictenotomidae resemble some cerambycids (Prioninae) but are not related to them. The Alleculidae (comb-clawed beetles) resemble some of the slender tenebrionids but are usually more active. Lagriidae (lagriids) have a characteristic shape, usually widened behind, and sometimes a metallic sheen. Colydiidae (cylindrical bark beetles), hard-bodied and shiny or roughened, may be cylindrical and somewhat flattened.

The Curculionidae (weevils (weevil)) range from slender to stout, elongated to egg-shaped; the bodies of some species contain many rounded projections (tubercles), those of others may be smooth or grooved. The mouth is located on the end of a snoutlike projection, which varies in shape from short and stout to long and slender and sometimes exceeds the length of the rest of the body. Some Anthribidae (fungus weevils (fungus weevil)), usually cylindrical in shape, have slender antennae that may be longer than the rest of the body; they are easily confused with the cerambycids. Brenthidae (primitive weevils) usually are long and slender with antennae projecting from the sides of the snout. Scolytidae (bark beetles (bark beetle), ambrosia beetles) do not have a distinct snout and are usually cylindrical in shape, as are Platypodidae.

Predators such as Carabidae (ground beetles) and Staphylinidae (rove beetles) help to control the populations of many insects by feeding on caterpillars and other immature insects (larvae), many soft-bodied adult insects, and insect eggs. Most of the Coccinellidae (ladybugs, ladybird beetles) are highly beneficial to man; both larvae and adults feed on plant-sucking insects (Homoptera) such as aphids and scale insects. Only a few coccinellids (e.g., Epilachna) feed on plants.

Most of the beetles and weevils harmful to man are phytophagous (plant feeders). Of primary importance are the leaf beetles (Chrysomelidae) and the weevils and their relatives (Curculionoidea). Leaf-beetle larvae feed on leaves, stems, or roots of plants; and most adults chew leaves. Various species of weevil larvae or adults have been found feeding on almost every plant part; especially numerous are species that bore into trunks, stems, and seeds. Both larval and adult forms of Scolytidae (bark beetles) are serious pests; they feed beneath tree bark, harming vital areas of living trees (e.g., cambium, pine needles, leaf stalks). The Platypodidae have similar habits, but the tropical Brenthidae (primitive weevils) usually attack deadwood.

The long-horned beetles (Cerambycidae) bore into stems, trunks, roots, and cones of living and dead trees and large semiwoody herbs; adults often feed on tender, new bark. Most pea-weevil larvae (Bruchidae) develop in dried seeds of leguminous plants (peas, beans). Buprestidae (metallic wood borers) have habits similar to those of cerambycids, and many kill trees or branches by boring in the cambium. The scarab beetles (Scarabaeidae) include many important pests of crop plants, lawns, and pastures. The larvae of many Melolonthinae (June beetles (June beetle), chafers (chafer)), for example, feed on grass roots. The Dynastinae (rhinoceros, unicorn (unicorn beetle), and elephant beetles) are often pests of palms, killing them by destroying the growing points. Lumber, furniture, and other items made from wood are sometimes severely damaged by several groups of beetles that bore in dry wood; e.g., powder-post beetles (Lyctidae), deathwatch beetles (Anobiidae (borer beetle)), and branch borers (Bostrychidae). Some anobiids also feed on various types of dried products; e.g., the cigarette beetle, Lasioderma serricorne, feeds on tobacco, various dried foods, and drugs.

Many groups of beetles function as scavengers, breaking down materials such as dead logs, lumber used in houses (in which case they are pests), dead plant and animal matter, excrement, and other waste products. Coleopterans that function as scavengers include Scarabaeidae, Tenebrionidae (darkling beetles), Silphidae (carrion and burying beetles (carrion beetle)), and Dermestidae (dermestid or hide beetles (dermestid beetle)). Some dermestid species cause serious damage in museums by feeding on dried animal materials. The larvae of several species of small dermestids damage carpets, upholstery, and clothing. However, some dermestids are valuable as scavengers; some of the carrion-feeding species (e.g., Dermestes caninus) are used by zoologists to clean skeletons of animals.

Little is known concerning the role of beetles in transmitting plant diseases. Since beetles do not suck plant juices as do plant-sucking insects (Homoptera), there is less likelihood of disease transmission. Transmission of diseases may occur, however, if beetles carry fungal spores on or in the body; e.g., the fungus that causes Dutch elm disease is transmitted by the European elm bark beetle, Scolytus multistriatus.

Reproduction is almost always bisexual in beetles, although some species are always parthenogenetic (reproducing without fertilization), and consist of females only. In some species parthenogenesis can sometimes occur. The male reproductive organ is a hardened tubelike structure called the aedeagus. The aedeagus enters a structure in the tip of the abdomen of the female (bursa copulatrix), and the sperm are stored in a saclike structure in the female (spermatheca) until they are needed to fertilize eggs. The females of most species lay eggs. After the larva has hatched, it feeds until its skin (cuticle) becomes too small and splits; the larva crawls out of the old skin (exuviae), and a new skin forms and hardens. The process, called molting, is repeated, usually from three to five times, until the larva is mature. During a nonfeeding period (prepupal stage) the insect enters the pupal stage. The pupa, which forms beneath the final larval skin and emerges when it splits, resembles the adult, except that it is soft and pale; in addition, the appendages are curled or loosely attached to the body, and the wings are folded in flat bags called wing pads. The adult emerges when the thin skin of the pupa is shed; the wings stretch out to full size, and the new outer skeleton hardens and becomes coloured. No further growth of the hardened skeleton occurs; the abdomen of the gravid (pregnant) female may enlarge, by stretching of membranes between the abdominal segments. The four developmental stages of Coleoptera—egg, larva, pupa, adult—constitute complete metamorphosis. The length of each stage in the life cycle depends on several factors; e.g., climate, nature of habitat, available food.

Eggs vary in form, may be laid singly or in groups, and usually are laid at a site that allows proper development of the larva—on a leaf of a host plant (leaf-eating species), in bark, or in tree trunks (wood-borers). Eggs also may be laid near roots, in flowers, in fruits, in tree injuries, on water plants, or under rocks.

There are several types of coleopteran larvae. Carabid larvae have a tapering, flattened, smooth body, as do those of staphylinids (rove beetles) and silphids (carrion beetles); larvae of the Dytiscidae (diving beetles), although somewhat similar to those of carabids, have a lobed air-float at the end. Larvae of click beetles (Elateridae) are cylindrical or flat and slender and have a hard surface; some click beetle larvae, called wireworms, feed on newly planted seeds and roots of plant crops (e.g., maize, cotton, potatoes); others feed in deadwood or on wood-boring beetle larvae (Cerambycidae). Larvae of Buprestidae (metallic wood borers), which are soft-bodied and slender, bore under the bark of trees or burrow beneath the surface of leaves.

Some beetles undergo hypermetamorphosis, in which they have different larval types in different instars (the stages between molts). The early larval stages usually are active, and the later stages are parasitic on other organisms. The active, young larvae of most Meloidae (blister beetles (blister beetle)), called triungulins, for example, hatch from eggs laid on flowers, become attached to bees visiting the flowers, and thus are carried to a bee nest, where they become parasitic on bee larvae.

Pupae of beetles usually have a form similar to that of the adult, except that the elytra are represented by pads on the exterior of the body; the colour, generally white, is sometimes pale brown or patterned. As the time for emergence of the adult approaches, the pupa may darken, especially the mandibles and eyes. After emerging from the pupal skin, the adult rapidly assumes its final adult form and coloration, although metallic colours may take some days to develop their final appearance.

Larvae that bore in wood, cones, or seeds and those that live in the ground or in excrement chew or dig a cavity, or pupal cell. In some cases, the pupa lies on a cushion of frass (chewed or torn wood fibres) or other material; in others, it is enclosed in a cocoon of frass or other material (e.g., a smooth, white, hard covering similar to the shell of a bird's egg). Sometimes material is used only to seal off the open end of the tube, gallery, or cell for protection from ants and other predators. After the body of the adult has hardened, the adult breaks or dissolves the barrier and emerges. Many wood borer adults must chew through solid wood to emerge, although the larva usually chews close to the surface of the tree, under the bark, before pupating.

The pupal stage lasts four days or longer. The life cycle from egg to adult requires 21 to 27 days in mild weather, longer in winter; adults may live more than 230 days. Females lay from 63 to 228 egg cases, with an average of about 3 eggs per case. There may be several generations per year.

In cooler temperate areas, life cycles may occupy much longer periods, even up to four years or more. In general, wood-boring beetles and root feeders have the longest life cycles, while leaf-feeding species have shorter ones. Several generations per year are possible with subtropical and tropical species.

Ecology, the relationship of organisms to each other and to their environment, represents fundamental interactions in nature. The habits of some of the small families of beetles have not yet been established.

In general, beetles are very well armoured insects and thus are reasonably protected against enemies; most, however, have parasites. Some tachinid flies, for example, lay their eggs on adult beetles, and the larvae feed inside their bodies. Beetle larvae also often have hymenopterous parasites; e.g., wasps. Wood-boring long-horned beetle larvae (Cerambycidae) are parasitized by wasps that lay their eggs directly on or in beetle larvae. Beetles are probably attacked by fewer predators than are many other insects; birds that often feed on various kinds of insects may not eat some kinds of beetles. Swifts and other birds, insectivorous mammals including bats, reptiles, frogs, and other insects may act as beetle predators. Some beetle predators feed particularly on beetle larvae, although many beetle larvae that feed on plants and in the ground probably are distasteful to birds and other predators.

Beetles are found in almost any habitat occupied by insects and feed on a variety of plant and animal materials. Many are predatory, some are scavengers, many are plant feeders (phytophagous), others feed on fungi, and a few are parasitic on other organisms. Beetles may live beneath the ground, in water, or as commensals in the nests of social insects such as ants and termites. Plant-feeding species may eat foliage, bore in wood or fruit, and attack roots or blossoms; any part of a plant may be a food source for some type of beetle. Many beetles eat stored plant or animal products, including various types of foods and clothing.

The Melolonthinae (June beetles, chafers) are phytophagous, the larvae usually feeding on roots of grasses or other plants, the adults feeding on leaves. The larvae of Rutelinae (shining leaf chafers (shining leaf chafer)), which have similar habits, sometimes feed in humus or in rotten wood. The adults of Cetoniinae feed on pollen and tree juices; the larvae, often called white grubs, feed on organic matter in the soil and may damage plant roots. The Dynastinae (rhinoceros, unicorn, and elephant beetles) feed in rotting wood, decaying vegetation, or humus. Buprestidae bore into living and dead trees, generally feeding on the cambium layer. Some of the small buprestids are leaf miners; that is, they feed on leaf cells between the upper and lower surfaces of a leaf. Moderate numbers of larvae and adults of Passalidae (bess beetles (bess beetle)), which feed on rotting logs, generally are found together; apparently the adults protect and feed their young, an unusual habit among beetles. The lucanids feed in logs and stumps as larvae; the adults often feed on juices from damaged trees, particularly at openings of larval tunnels of Cerambycidae (long-horned wood-boring beetles).

Tenebrionidae (darkling beetles) live in various habitats as scavengers or predators. Those in rotten wood and under bark may be predatory on other larvae; those in damp places may feed on rodent excrement, on other wastes, or on fungi; and those in stored products feed on grain, meal, and other staples. Lagriidae (lagriids) and Alleculidae (comb-clawed beetles) feed in rotten logs. Melandryidae (false darkling beetles) usually feed on fungi or in old wood. Pythids usually are scavengers in burrows of other beetles, including weevils. Rhipiphoridae (wedge-shaped beetles), which usually are parasites in wasps' nests and undergo hypermetamorphosis, are related to another group of insects, the Strepsiptera, which are mostly parasitic in the bodies of wasps, leafhoppers, grasshoppers, and other insects. Some larvae of Mordellidae (tumbling flower beetles (tumbling flower beetle)) may live in dead or dying deciduous wood or attack the heartwood of weak trees; others may be found in pith or herbaceous weeds. The adults frequent flowers and are good fliers. Meloidae (blister beetles) undergo hypermetamorphosis and usually live in bee nests, feeding on eggs and stored food. The larvae of Pyrochroidae (fire-coloured beetles) live under bark and in rotting wood. Most cerambycid larvae are wood borers; a few live in large herbs or in cones, and many feed in roots. The adults, which chew new bark or other plant parts, at times cut rings in branches, killing them so that the larvae can bore in. Some chrysomelid larvae eat leaves; many others are root feeders. Members of one group of chrysomelid larvae (Sagrinae) bore in stems of leguminous vines, causing gall-like swellings; others (Donaciinae) feed on and in the stems of freshwater plants, often below the water level, or hide (Hispinae) in unopened parts of monocot plants such as palms, grasses, bamboos, and gingers; still others (Alticinae) may feed on both roots and leaves or act as leaf miners. Bruchidae live primarily in the seeds of legumes; hence their common name, seed beetles (or pea weevils).

Weevils also are diverse in habits. Many larvae bore into solid wood of living or dead trees and stumps; some feed on or in roots or in stems of semiwoody plants; and others feed in seeds, pods, grain, meal, fruits, nuts, and other parts of plants; some are hidden inside the plants. Most adults are able to fly; nonflying forms, however, are more abundant on small oceanic islands than are flying forms. Some species, which develop in rotting stumps, soil, or palm trunks, make a rough cocoon from frass. Brenthids develop in dead trunks and stumps; anthribids live in various parts of plants, often in seeds; and scolytids feed largely under the bark of living trees, although some bore in other parts of plants, seeds, cones, and needles.

Certain beetles have special ecological relationships. Beetles that live in nests of ants and in nests of termites have become modified in form because certain structures that are no longer needed have degenerated (e.g., wings, wing covers); some have almost lost the power of locomotion. Some species have evolved glands that produce secretions attractive to the host ants or termites.

An association of a different type is known involving beetles in the mountains of New Guinea; called epizoic symbiosis, the association occurs on the backs of large leaf-feeding weevils found on Nothofagus and other trees in the moss forests. Various kinds of algae, fungi, lichens, liverworts, mosses, and diatoms develop on the backs of the weevils. Among them live protozoans, rotifers, nematodes, phytophagous mites (mite), and parasitic mites. The phytophagous mites, known only from this association, feed primarily in the fungal growth on the backs of the beetles. Plant spores, which may be carried from one weevil to another either by the mites during mating of the weevils or by air dispersal, are trapped with a sticky fluid that may be produced by the weevils for this purpose.

Ambrosia beetles (Scolytidae) associate with fungi in the host tree. Certain adult scolytids and platypodids have specialized structures called mycetangia, which are used to carry the fungi when the beetles seek out new host trees.

As in all adult insects, the segmented body consists of three primary body regions: head, thorax, and abdomen. In beetles, however, two of the three thoracic segments (mesothorax and metathorax) are attached to the abdomen; the third (prothorax), isolated as the region between the head and trunk, is covered by a dorsal plate, the pronotum. The body covering (exoskeleton) varies from very horny and rigid to soft and flexible, but it usually consists of hard plates (sclerites) separated by flexible membranes.

The antennae are usually 11-segmented but vary widely in form. The jaws (mandibles) may be relatively large, in some as long as the rest of the body, or almost completely absent; usually they are triangular in shape and suitable for biting or chewing. The paired maxillary and labial palps are usually small and are used for feeding or handling food, but in some beetles one or the other pair may be greatly enlarged. The compound eyes are usually prominent, but are sometimes reduced or absent, and occasionally divided. Simple eyes (ocelli) are rarely present. A neck is sometimes evident, but in many beetles the head is recessed into the prothorax or under the pronotum.

The prothorax is generally very distinct; the mesothorax and metathorax are hidden under the elytra along with most of the abdomen. The pronotum may be four-sided very wide or very long, and sculptured with lateral spines or dorsal grooves and pits. The front legs emerge from cavities in the underside that may be confluent or separated by other parts. The mesothoracic spiracle (respiratory opening) is often visible just behind the base of the front legs. The mesothorax bears the elytra (wing covers) and the second pair of legs. The metathorax bears the flying wings (hindwings) and the third pair of legs.

The legs are modified in various ways, for running, swimming, jumping, digging, or clasping. In some beetles, wings are not capable of producing flight, but in many others, they are powerful and sustain strong flight.

The abdomen is composed of nine or ten segments, but often some of these are not externally visible. From five to eight segments can usually be seen, with short apical appendages evident in some beetles. Each abdominal segment has a pair of spiracles, the openings into the air-tube (tracheal) system. Predatory beetles generally have short digestive tracts. Differences in salivary glands occur, depending on the food source.

Coleoptera larvae differ in appearance from adults. This is characteristic of insects with complete metamorphosis (Endopterygota), in which the wings develop internally until they become apparent in the pupal stage. The differences in body form of the larvae are closely associated with larval habitats and modes of feeding. The numerous predatory types, for example, have slender or gradually tapered bodies, with large, slender, mandibles, and relatively long legs; thus, these larvae are adapted to be rapid runners that capture and hold prey with their mandibles. A mouthpart structure (epipharynx) may be adapted for imbibing body fluids that exude from wounds caused by the mandibles. Typical predatory larvae are found in the dytiscids (predacious water beetles) and carabids (ground beetles). The larvae of tiger beetles (tiger beetle) (Cicindelidae), although similar to the dytiscid type, live in holes in the ground or in branches and capture passing insects. The enlarged head of the tiger beetle larva fills the burrow opening, and its legs are modified for attachment and leverage. Haliplid, staphylinid, and gyrinid larvae are also similar to those of dytiscids, except that the gyrinids have a gill on the side of each abdominal segment, rather than at the tip of the abdomen, as do most of the others. Some predatory larvae are generally less tapered, sometimes less armoured, and have shorter appendages than do the dytiscids; e.g., larvae predatory on other larvae in deadwood or in the ground. Larvae such as those of the Histeridae (hister beetles (hister beetle)), which usually live in special environments such as dung and tunnels in wood, have short appendages but slender mandibles.

Another larval type includes many of the scavengers; e.g., silphids, hydrophilids. These larvae have short legs and mandibles.

Wood-feeding borer larvae (cerambycid and buprestid type) have soft white bodies that may be cylindrical or flattened. The thoracic region of buprestids is flat and broad, the head is dark and retracted in the thorax, and the mandibles are short and stout. Larvae of wood-boring beetles usually have yeast associated with the digestive tract that helps to convert wood to digestible compounds.

Larvae that feed on leaves, stems, and roots (chrysomelid type) are short and oval shaped. Coccinellid larvae are similar in form except for longer legs, and prey on soft-bodied insects like aphids.

The lamellicorn larval type is C-shaped, has a soft body, and a hard, dark, nonretractile head. These larvae usually are found in protected habitats, where they feed on roots, rotten wood, or excrement. Weevil larvae have a similar form, although the head may be a little smaller, the body less arched. Another larval type is that of the elaterids and many tenebrionids, which have very slender usually brown bodies, with a hard outer skeleton.

Certain beetles, especially those living in ants' nests, resemble ants, and the common wasp beetle of Europe, Clytus arietis, both in its movements and coloration, closely resembles a wasp.

Some beetles obtain some measure of protection possibly from their repellent appearance or from their evil-smelling or distasteful secretions, either in the form of exudations of blood from definite parts of the body or as the product of special fetid glands. The so-called bombardier beetles of the Carabidae have the property of secreting an evil-smelling defensive fluid from the anal end of the body. In some cases, this fluid volatilizes explosively into a gas at high temperature when it comes into contact with the air; it acts as a repellent to other insects or enemies. A number of beetles secure protection by virtue of their agility; many ground beetles and tiger beetles run rapidly, and the latter also readily take flight. The flea beetles (flea beetle) (a group of the Chrysomelidae) have remarkable powers of leaping.

Many beetles produce sound, usually by rubbing one part of the body (a scraper) against another part (the file). These stridulating organs are generally present in both sexes and probably serve for mutual sex calling. Some beetles have a filelike area on the head that is rasped by the front margin of the prothorax. Among the cerambycids, sound is produced either by rubbing the rear margin of the prothorax over a grooved area on the mesothorax or by rubbing the femurs of the hind legs against the margins of the elytra.

Stridulation, however, is not confined to adult beetles; it occurs also in certain larvae. Some larvae of the Scarabaeoidea, for example, have a series of ridges, or tubercles, on the coxal segment of the middle pair of legs, and the hind legs are modified in various ways as rasping organs. In the larvae of some chafers (Melolonthinae), a ridged area on the mandible is rasped by a series of teeth on the maxillae. Stridulation in larvae is independent of sex and may be used to warn neighbouring larvae to avoid getting in each other's way.

Some beetles emit a bright light, whose source is in special luminous organs that consist of an outer, light-producing layer and an inner reflector layer. The outer layer is supplied with oxygen by means of air tubes (tracheae), and the reflector layer contains crystals that apparently act as a background, scattering the light and preventing its dispersion internally. The light is produced during a reaction involving a compound (luciferin) and an enzyme-like substance (luciferase) in the outer layer of the luminous organ. Luminous beetles include the Lampyridae (fireflies), Phengodidae, Drilidae, and certain Elateridae (click beetles). A familiar example of a firefly is the common European glowworm (Lampyris noctiluca), whose wingless female emits a bright light near the hind end of the body; the winged male emits a much feebler light. Both sexes of the elaterid genera Pyrophorus and Photophorus are winged and luminous.

Coleopterans are very ancient insects; they date from the Permian Period (about 225,000,000–280,000,000 years ago), after the appearance of gymnosperm plants. Although the beetles have a number of similarities to another ancient group of insects, the cockroaches (Blattaria), they probably evolved from ancestors of the present-day Neuroptera. This theory is based largely on the nature of the life cycle of beetles and on their larval structure. Although many beetle fossils are known, they consist mostly of isolated elytra, which reveal little about the history of the order. Complete fossil specimens are closely related to living forms. The evolution of elytra may have been associated with the habit of living under bark of trees, where protection for flying wings is required. Most of the insects that live under bark are beetles.

One distinctive feature of coleopterans is wing structure. Most beetles have two pairs of wings. The front pair, which may be thickened, leathery, or hard and brittle, are called elytra and usually serve only as protective covers. A few beetles have greatly reduced wings. Variations in the structure of the first abdominal segment is one criterion used to separate the various suborders of Coleoptera; the hind coxal leg segments (by which the legs are attached to the body) may divide the abdominal segment partially or completely. Sometimes the abdominal segments are fused, the articulations marked by form sutures.

Variation in length, texture, and appearance of elytra, as well as the number of abdominal segments exposed by short elytra, are used to distinguish the various superfamilies. Characters associated with the size and shape of the coxae also are used as distinguishing features. Structure of antennae and legs are important considerations for taxonomic criteria, as are larval structure, head structure (including mandibles, or jaws), pattern of veins in wings, habitats, and behaviour.

About 135 families of beetles are known, of which 120 occur in the Western Hemisphere.

There have been a number of different classifications of Coleoptera. Many were based on the suborders Adephaga and Polyphaga; the latter, which contains about 90 percent of the beetles, included a number of divisions (e.g., clavicorns, serricorns, lamellicorns, phytophagous beetles, and weevils). Sometimes these divisions are considered as superfamilies or series, and sometimes (particularly weevils and relatives) as suborders. The classification below is based on that of R.A. Crowson (1955); it includes four suborders.

There are many different opinions among coleopterists concerning the relationships of the various groups of beetles; the groups that should be given family status; and the placement of families in superfamilies and suborders. Little information is available about many coleopteran groups, so that their taxonomic affinities are uncertain. Although Crowson includes Strepsiptera (twisted-wing parasites) as a superfamily of Coleoptera, opinion is divided concerning their status, and they have not been included here. Some classifications of Coleoptera recognize three suborders, Archostemmata, Adephaga, and Polyphaga, rather than the four used by Crowson.